1. Field of the Invention
The present invention relates to a serial-parallel battery module control circuit, and more particularly to a rechargeable lithium battery protection device.
2. Description of Prior Art
In general, rechargeable batteries such as nickel-cadmium (Ni—Cd) batteries and nickel-hydrogen (Ni-MH) batteries are popular in many electrical applications. The Ni—CD battery is a pervious-generation product and has been phased out from the market. The major disadvantage of the Ni—Cd battery resides on that the Ni—Cd battery has a memory effect and comes with a short life. Furthermore, cadmium is a heavy metal which may cause environmental contamination easily. Therefore, a Ni—MH battery was developed to overcome the shortcomings of the Ni—Cd battery by reducing or eliminating the memory effect of the Ni—Cd battery, improving the capacitance, resisting an overcharge or an overdischarge, extending the life of use, and shortening the charging time. Mostly important, the poisonous heavy metal is not used and thus greatly reducing the environmental contamination problem. Of course, the Ni—MH battery is not perfect either, and its adaptability to the environment is low, and thus it cannot be used at a temperature above 45° C. or below 0° C. Therefore, the Ni—MH battery cannot supply power to electronic equipments at an extremely hot or extremely cold place. Further, if the Ni—MH battery is not used for a while after being fully charged, the power will be lost substantially, and thus the battery requires a recharge.
In view of above, a rechargeable lithium battery is developed to provide more advantages. However, if the rechargeable lithium battery is charged with an over-voltage current, there will be a risk of an explosion of the battery. On the other hand, if the rechargeable lithium battery is discharged with an excessively low voltage, and the rechargeable lithium battery is lower than a basic voltage, the life of use of the rechargeable lithium battery will be affected adversely. Therefore, the rechargeable lithium battery generally adds a protection device as shown in FIG. 1 for its use (during charging or discharging).
In FIG. 1, four pieces of rechargeable lithium batteries are used for the illustration of the invention, and the protection device 100 comprises four batteries connected in series for a special use of a protection IC 101 and a circuit switch 103. In an actual design, an appropriate protection IC 101 is selected according to the number of serially connected rechargeable lithium batteries 102. If the protection IC 101 is charged or discharged, the voltage of each rechargeable lithium battery 102 will be detected. If an abnormal voltage of the rechargeable lithium battery 102 is detected, the protection IC 101 will send a signal to the circuit switch 103 to disconnect the circuit switch 103 and cut off a charging current or a discharging current. If the number of the serially connected rechargeable lithium batteries 102 varies, it is necessary to design and make a new protection device 100. Furthermore, most manufacturers provide at most four serially connected rechargeable lithium batteries 102 for the protection ICs 100, because the more the rechargeable lithium batteries 102, the more complicated is the circuit design of the protection IC 101. The complexity of the circuit and the aging of the battery may create an unbalanced problem, and thus the protection circuit for five or more serially connected rechargeable lithium batteries is difficult to make.